Danfoss VLT AQUA Drive FC 202 Design Manual page 18

Product Overview
2.1.3 Example of Energy Savings
2 2
As shown in Figure 2.2, the flow is controlled by changing
the pump speed, measured in RPM. By reducing the rated
speed by only 20%, the flow is also reduced by 20%. This
is because the flow is directly proportional to the speed.
The consumption of electricity, however, is reduced by up
to almost 50%.
If the system only has to supply a flow that corresponds to
100% a few days in a year, while the average is below 80%
of the rated flow for the remainder of the year, the energy
savings are even greater than 50%.
Figure 2.2 describes the dependence of flow, pressure, and
power consumption on pump speed in RPM for centrifugal
pumps.
100%
80%
50%
25%
12,5%
Figure 2.2 Affinity Laws for Centrifugal Pumps
Q n
1 1
Flow : =
Q n
2 2
H
1
Pressure : =
H
2
P n
1 1
Power : =
P n
2 2
Assuming an equal efficiency in the speed range.
Q=Flow
Q =Flow 1
1
Q =Reduced flow
2
H=Pressure
H =Pressure 1
1
H =Reduced pressure
2
Table 2.1 Affinity Laws
16
Flow ~n
3
Power ~n
50% 80%
n 2
1
n
2
3
P=Power
P =Power 1
1
P =Reduced power
2
n=Speed regulation
n =Speed 1
1
n =Reduced speed
2
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VLT AQUA Drive FC 202
2.1.4 Valve Control versus Speed Control of
Valve control
As the demand for process requirements in water systems
varies, the flow has to be adjusted accordingly. Frequently
used methods for flow adaptation are throttling or
recycling using valves.
A recycle valve that is opened too wide can cause the
pump to run at the end of the pump curve, with a high
flow rate at a low pump head. These conditions do not
only cause a waste of energy due to the high speed of the
pump, but can also lead to pump cavitation with resultant
pump damage.
Throttling the flow with a valve adds a pressure drop
across the valve (HP-HS). This can be compared with
accelerating and pulling the brake at the same time in an
attempt to reduce car speed. Figure 2.3 shows that
throttling makes the system curve turn from point (2) on
the pump curve to a point with significantly reduced
efficiency (1).
Pressure ~n 2
n
100%
Centrifugal Pumps
MG20N622